The invention relates to a filter unit for connectors comprising:
at least one substrate made of electrically insulating material and having two mutually opposite sides and through openings for contact elements of the connector;
capacitors situated on at least one of the two opposite sides of the substrate in the region of the through openings, each of said capacitors comprising at least one first conducting layer in contact with the substrate, a dielectric layer and a second conducting layer; and
at least one passivating, electrically insulating cover layer disposed above each of said capacitors, said cover layer comprising at least one first flexible passivating, electrically insulating layer having a low moisture absorption coefficient, and a second hard layer.
Such a filter unit is disclosed by European Patent Application EP-A-0 124 264.
In electrical transmission engineering, pulse-type signals are increasingly being used to transmit information. As known in electrical engineering, pulse-type signals can be broken down into a series of sinusoidal signals having increasing frequency, the so-called higher harmonics. In the case of signals having a high pulse frequency, as is usual in the case of computers, higher harmonics may occur in the megahertz region or even up into the gigahertz region.
The steepness of the pulse edges, also referred to as the rise time, likewise plays an important role. A normal rise time of one nanosecond corresponds to a higher harmonic frequency of approximately 350 MHz, regardless of the pulse frequency itself.
Such higher harmonics are found to be very troublesome. In a space in which various interconnected electronic processing apparatuses which generate pulse-type signals are present, interferences readily occur in the information processing as a consequence of the higher harmonic signals. Such interferences may assume a proportion such that the correct functioning of, for example, computers is no longer possible.
In order to be able to keep the total interference level as low as possible, it is necessary to use filters with which the undesirable higher harmonic frequencies can be attenuated without the desired information signal being excessively distorted. A capacitor is a component which is suitable for this purpose because its reactance is inversely proportional to the frequency. That is to say, the reactance is greater for relatively low frequencies than for higher frequencies.
European Patent Application 0,124,264, referred to above, discloses a filter unit for connectors in which two cover layers may be used on the capacitors. An inner cover layer covering the capacitor is composed of glass and an outer cover layer covering the inner cover layer is made of silicone. This known combination of cover layers is unsuitable for fairly large surface areas because, in that case, the mechanical stresses in the glass become too high with increasing temperature owing to the different temperature coefficients and cracks may occur. Because the outer layer is composed of silicone, which has a high moisture diffusion coefficient, moisture is then able to penetrate the capacitors via the two cover layers.
In another filter unit, as described in EP-A-0,299,563, each of the contact devices of a connector is decoupled to earth (chassis) with the aid of a capacitor. The filter unit is manufactured by the so-called thick-film screen-printing technology on a flat substrate. Such methods permit capacitors to be inexpensively manufactured which have an inductance low enough to attenuate signals effectively at high frequencies. The capacitance value of the flat capacitors produced in this way is directly proportional to the surface area of the mutually opposite electrodes and the relative permittivity of the dielectric material present in between, but is inversely proportional to the distance between the electrodes.
In the filter unit, known from EP-A-0,299,563 the capacitors are covered with a protective cover layer, which may be made up of various sublayers. Previously, silicone, which is flexible and has a low moisture absorption coefficient and a high moisture diffusion coefficient, has been used for all these layers. As a result of its flexibility, silicone is very suitable for use over fairly large surface areas because it readily adapts to the expanded form of the filter unit at higher temperatures. Usually such filters have to be washed after manufacturing by a suitable solvent. Previously used solvents, however, affect the ozone layer and, therefore, alternative solvents are needed. Such an alternative is a deionized water solution. However, it appears that silicone is not sufficiently resistant to deionised water solutions used in washing, for example, those provided with 10% Lonco Terge 520 D detergent. If washing is carried out with this solvent, moisture can penetrate the capacitor, with short circuiting or a low dielectric breakdown voltage as a result.
In US-A-5,130,780 a dual in-line packaging is described which comprises a substrate with a first cover layer of a first material covering the bottom surface of the substrate and a second cover layer of a second material covering the top surface of the substrate. The first material is resilient and moisture resistant and the second material is hard compared to the first material. During manufacturing of the known device, the first layer is first applied to the bottom surface and partly to the peripheral surface of the packaging; next, the second layer is applied to the top surface and, again, partly to the peripheral surface, thus producing a cover layer at the peripheral surface that comprises two sub-layers: a first sub-layer that is resilient and moisture resistant and a second sub-layer that is hard compared to the first sub-layer. Since by a suitable choice of the first and second materials a good adherence between them occurs at the peripheral surface where they overlap moisture is also prevented from entering the inside of the packaging via the junction interface between the first and second layer at the peripheral surface. However, in the known device the overlap of the first and second layers at the peripheral surface is only established because of preventing moisture to enter the device via the interface between the first and second layers. The bottom surface is entirely covered by one cover layer, i.e., a resilient layer, whereas the top layer is also entirely covered by one layer, i.e., a hard layer. Therefore, the known device is also not sufficiently resistant to deionised water solutions used in washing; especially, moisture can penetrate into the known device through the first cover layer at the bottom surface during washing with deionised water.
The object of the invention is to provide a filter unit of the type mentioned at the outset, which is completely moisture- and contamination-repellent, even if washing is carried out with deionised water.